The present invention relates to a novel resin composition for cationic electrodeposition coating and, more particularly, it relates to a resin composition for cationic electrodeposition coating which exhibits prominent properties in higher film building, curing process and retaining film building during the operation, has little heating loss, exhibits excellent smoothness and physical properties and shows good chemical performance in terms of corrosion resistance and solvent resistance.
As compared with air spraying and electrostatic spraying, an electrodeposition has better throwing power and results in less environmental pollution when applied to materials having recessed area such as automobiles and electric devices and, therefore, it has been widely put to practical use as a primer coating. However, various kinds of solvents are used therefor with an object of assuring the workability, particularly film thickness of the coating, at a coating job site.
Problems to be Solved by the Invention!
When, however, the recent trend in environmental regulation is taken into consideration, it is desired that the amount of organic solvents in the electrodeposition coating is as little as possible (i.e. making a VOC lower).
In view of the above, the present inventors have conducted an intensive study for developing a resin composition for cationic electrodeposition coating which can be well baked at the temperature of 150-170.degree. C., exhibits prominent properties in higher film building and retaining film building during the operation, has little heating loss, exhibits excellent smoothness and physical properties and shows good chemical performance in terms of corrosion resistance and solvent resistance. It has been found that said objects can be achieved by the use of a blocked polyisocyanate in which a part of isocyanate groups is polymerized with a polyol and which is simultaneously blocked by two kinds of blocking agents, i.e. (1) a blocking agent represented by a formula R--(OC.sub.2 H.sub.3 X).sub.n --OH (wherein R is an alkyl having 1-6 carbon atom(s); X is H or methyl; and n is 1-6) and having a molecular weight of not less than 150 and (2) another blocking agent having a deblocking temperature of not higher than about 150.degree. C.
Thus, the present invention relates to a resin composition for cationic electrodeposition coating containing (A) an epoxy resin (a base resin) having cationic groups and (B) a blocked polyisocyanate (a curing agent), characterized in that, a blocked polyisocyanate in which a part of the isocyanate groups is polymerized with a polyol and which is blocked by (1) a blocking agent represented by a formula R--(OC.sub.2 H.sub.3 X).sub.n --OH (wherein R is an alkyl having 1-6 carbon atom(s); X is H or methyl; and n is 1-6) and having a molecular weight of not less than 150 and (2) another blocking agent having a deblocking temperature of not higher than about 150.degree. C. is used.